Ferroelastic Nanodomain-mediated Mechanical Switching of Ferroelectricity in Thick Epitaxial Films

Artificial Domain Patterning in Ultrathin Ferroelectric Films via Modifying the Surface Electrostatic Boundary Conditions

Nanoscale spatially controlled modulation of the properties of ferroelectrics via artificial domain pattering is crucial to their emerging optoelectronics applications. New patterning strategies to achieve high precision and efficiency and to link the resultant domain structures with device functionalities are being sought. Here, we present an epitaxial heterostructure of SrRuO3/PbTiO3/SrRuO3, wherein the domain configuration is delicately determined by the charge screening conditions in the SrRuO3 layer and the substrate strains. Chemical etching of the top SrRuO3 layer leads to a transition from in-plane a domains to out-of-plane c domains, accompanied by a giant (>105) modification in the second harmonic generation response. The modulation effect, coupled with the plasmonic resonance effect from SrRuO3, enables a highly flexible design of nonlinear optical devices, as demonstrated by a simulated split-ring resonator metasurface. This domain patterning strategy may be extended to more thin-film ferroelectric systems with domain stabilities amenable to electrostatic boundary conditions.

Ferroic Phases in Two-Dimensional Materials

Two-dimensional (2D) ferroics, namely ferroelectric, ferromagnetic, and ferroelastic materials, are attracting rising interest due to their fascinating physical properties and promising functional applications. A variety of 2D ferroic phases, as well as 2D multiferroics and the novel 2D ferrovalleytronics/ferrotoroidics, have been recently predicted by theory, even down to the single atomic layers. Meanwhile, some of them have already been experimentally verified. In addition to the intrinsic 2D ferroics, appropriate stacking, doping, and defects can also artificially regulate the ferroic phases of 2D materials. Correspondingly, ferroic ordering in 2D materials exhibits enormous potential for future high density memory devices, energy conversion devices, and sensing devices, among other applications. In this paper, the recent research progresses on 2D ferroic phases are comprehensively reviewed, with emphasis on chemistry and structural origin of the ferroic properties. In addition, the promising applications of the 2D ferroics for information storage, optoelectronics, and sensing are also briefly discussed. Finally, we envisioned a few possible pathways for the future 2D ferroics research and development. This comprehensive overview on the 2D ferroic phases can provide an atlas for this field and facilitate further exploration of the intriguing new materials and physical phenomena, which will generate tremendous impact on future functional materials and devices.

Real-space imaging of periodic nanotextures in thin films via phasing of diffraction data

New properties and exotic quantum phenomena can form due to periodic nanotextures, including Moire patterns, ferroic domains, and topologically protected magnetization and polarization textures. Despite the availability of powerful tools to characterize the atomic crystal structure, the visualization of nanoscale strain-modulated structural motifs remains challenging. Here, we develop nondestructive real-space imaging of periodic lattice distortions in thin epitaxial films and report an emergent periodic nanotexture in a Mott insulator. Specifically, we combine iterative phase retrieval with unsupervised machine learning to invert the diffuse scattering pattern from conventional X-ray reciprocal-space maps into real-space images of crystalline displacements. Our imaging in PbTiO3/SrTiO3 superlattices exhibiting checkerboard strain modulation substantiates published phase-field model calculations. Furthermore, the imaging of biaxially strained Mott insulator Ca2RuO4 reveals a strain-induced nanotexture comprised of nanometer-thin metallic-structure wires separated by nanometer-thin Mott-insulating-structure walls, as confirmed by cryogenic scanning transmission electron microscopy (cryo-STEM). The nanotexture in Ca2RuO4 film is induced by the metal-to-insulator transition and has not been reported in bulk crystals. We expect the phasing of diffuse X-ray scattering from thin crystalline films in combination with cryo-STEM to open a powerful avenue for discovering, visualizing, and quantifying the periodic strain-modulated structures in quantum materials.

Domain Switching in BaTiO3 Films Induced by an Ultralow Mechanical Force

Low-energy switching of ferroelectrics has been intensively studied for energy-efficient nanoelectronics. Mechanical force is considered as a low-energy consumption technique for switching the polarization of ferroelectric films due to the flexoelectric effect. Reduced threshold force is always desirable for the considerations of energy saving, easy domain manipulation, and sample surface protection. In this work, the mechanical switching behaviors of BaTiO₃/SrRuO₃ epitaxial heterostructure grown on Nb:SrTiO₃ (001) substrate are reported. Domain switching is found to be induced by an extremely low tip force of 320 nN (estimated pressure ∼0.09 GPa), which is the lowest value ever reported. This low mechanical threshold is attributed to the small compressive strain, the low oxygen vacancy concentration in BaTiO₃ film, and the high conductivity of the SrRuO₃ electrode. The flexoelectricity under both perpendicular mechanical load (point measurement) and sliding load (scanning measurement) are investigated. The sliding mode shows a much stronger flexoelectric field for its strong trailing field. The mechanical written domains show several advantages in comparison with the electrically written ones: low charge injection, low energy consumption, high density, and improved stability. The ultralow-pressure switching in this work presents opportunities for next-generation low-energy and high-density memory electronics.

Mechanical Polarization Switching in Hf0.5Zr0.5O2 Thin Film

HfO₂-based films with high compatibility with Si and complementary metal-oxide semiconductors (CMOS) have been widely explored in recent years. In addition to ferroelectricity and antiferroelectricity, flexoelectricity, the coupling between polarization and a strain gradient, is rarely reported in HfO₂-based films. Here, we demonstrate that the mechanically written out-of-plane domains are obtained in 10 nm Hf_0.5Zr0_.5O₂ (HZO) ferroelectric film at room temperature by generating the stress gradient via the tip of an atomic force microscope. The results of scanning Kelvin force microscopy (SKPM) exclude the possibility of flexoelectric-like mechanisms and prove that charge injection could be avoided by mechanical writing and thus reveal the true polarization state, promoting wider flexoelectric applications and ultrahigh-density storage of HZO thin films.